KATHMANDU/BEIJING (REUTERS)

Nepal's rescue teams, helped by Chinese and Indian experts, stepped up efforts on Monday to reach hundreds of people ​believed to be stuck inside blocked hydropower project tunnels in the aftermath of last week's Himalayan flood that carved a trail of destruction in the country's valley towns and villages as well as across the border in China.

The unprecedented deluge of ice, rock, mud and debris on the Nepal-China border on Wednesday - blamed on a glacier collapse - killed more than 900 people and nearly 5,000 are still ⁠missing.

Hundreds of bodies, many unidentified, have been buried in shallow graves as they began decomposing in the humid weather in the plains of the Himalayan country after ​being swept down the mountains by the torrent.

Nepali officials said that the focus was on reaching the 933 workers believed to be trapped at 11 ​hydropower projects, including many who ‌were working in about half a dozen tunnels, in the two hard-hit districts of Rasuwa ⁠and Nuwakot.

Pictures and ​videos shared by rescue teams showed earth-moving equipment digging and shovelling mud and rocks under lights in the dark of the night as workers watched. Soldiers used torchlight in a big pit they had created to look for space to get into one tunnel.

The Nepali army said search-and-rescue teams were using controlled blasts, excavators and searchlights to clear water and mud and reach trapped people. Teams recovered three bodies after entering two tunnels, while other sections remained buried in mud and were still being searched, it ‌said.

"A large amount of debris has been deposited and that is posing us a big challenge to open the tunnels,” Nepal ‌army spokesperson Raja Ram Basnet told Reuters. "Our main focus is on opening the tunnels.”

On Monday, Nepali authorities said 903 people had died, with 4,247 missing, including the 933 at hydropower projects. On the Chinese side, authorities said 16 people were killed in Gyirong County and 546 were missing.

The Red Cross has estimated that more than 90,000 people ​are likely to have been affected by the disaster, which China has linked to the effects of climate change.

There has been a boom in hydropower projects in Nepal since the turn of the century as the mountainous country sought to take advantage of its Himalayan water resources to address chronic domestic power shortages and exploit prospects of selling electricity to southern neighbour India.

Hydropower projects in Nepal's mountains need tunnels to carry water through the steep terrain, allowing developers to exploit the sharp elevation drop between Himalayan rivers and powerhouses to generate electricity.

Prime Minister Balendra Shah said the flood was an "unprecedented and devastating natural disaster” and ‌it remained challenging to compile a complete assessment of the damage.

Beijing has said ​261 foreign nationals from 23 countries were unaccounted for in Tibet, near a key border crossing with Nepal.

"Currently, China is making every effort to search for and rescue missing persons both domestically and internationally, while maintaining close communication with foreign embassies ​in China to jointly conduct a thorough investigation of missing foreign nationals," Guo added.